There are several known methods to detect the presence and measure the concentration of combustible gases and vapors (collectively referred to herein as “combustible gases” or “combustibles”). In one such method, the combustible gases are detected based on the actually measuring the rate of heat liberation during catalytic combustion. A combustible gas detector detects and measures combustible gases by burning a gas sample on a catalytic sensor. Catalytic combustion occurs in the sensor on a surface of a heated porous substrate, e.g. silica or alumina that has been impregnated with a catalyst. The resulting increase in temperature of the substrate is proportional to the rate of heat generation during the catalytic combustion and is measured electronically by sensing a resistance change of an imbedded resistance temperature detector (RTD).
A known type of catalytic combustible gas detector has a catalytic bead sensor, a reference bead, and a Wheatstone bridge. The bead sensor conventionally includes a catalyst-impregnated substrate in a form of a small bead, an RTD and a heater combination made as a tiny Platinum (Pt) wire coil imbedded in the body of the bead. Typically, the reference bead (also referred to as a compensating bead) is similar to the catalyst-impregnated bead with a heater, except that the compensating bead is not impregnated with a catalyst. The sensing and reference beads are supported by the Pt wires, which are the extensions of the heating coils. The catalyst impregnated bead and reference bead may be shielded from ambient gas by a porous cup (enclosure). The cup enclosure is permeable to the gas so as to allow ambient gas to reach the beads, essentially by means of diffusion. The cup may be formed out of stainless steel gauze or a more robust porous material, e.g. sintered ceramic. A porous ceramic cup may also serve as a flame-retarding barrier to explosion proof the detector design.
Conventional catalytic sensors are exposed to a continual influx of combustible gas that diffuses toward the beads from the environment. The sample gas may also reach the beads by convection. Gas velocity variations around the sensor in this case may provoke false sensor readings. Prior attempts to reduce measurement error due to gas velocity variations tend to diminish the sensor sensitivity and prolong the response time of the sensor.
Catalytic sensing beads and reference beads of conventional sensors are typically connected as the resistive shoulders of a balanced Wheatstone bridge circuit. Combustion heating on the sensing bead causes the resistance of the RTD imbedded into sensing bead change with respect to the resistance of the reference bead RTD to create a bridge misbalance.
The output voltage of the bridge is indicative of the resistive misbalance between the sensing and reference RTDs and is output as a signal from which concentration of combustible gas(es) is derived. However, the bridge misbalance in a conventional detector may be influenced by factors other than the catalytic combustion on the sensing bead, including: aging of the beads, changes in the background such as variations in ambient temperature, non-combustible gas mixture composition, and radiation absorption in the vicinity of the sensor beads due to different moisture concentrations. These other factors may significantly raise the lower detection limit of the detector, and cause the response of the detector to drift out of its rated detection level or range for combustible gases.
The sensitivity to combustible gases of a conventional catalytic detector may also be reduced due to “poisoning” of the catalyst in the sensor bead. When poisoned, sensors become less sensitive to combustibles. Reduced sensitivity to combustible gases and sensor drift are troublesome for a catalytic detector, especially for those used in critical applications, such as lower explosive limit (LEL) detectors that are employed to prevent fire and explosions.
“Poisoning” of catalytic bead sensors is conventionally detected by direct application of a gas sample with a known concentration of combustibles. With diffusion type sensors, this procedure is relatively cumbersome. Moreover, long periods of time, e.g. up to several months, may pass between when the detector looses sensitivity and when the sensitivity loss in the detector is discovered. A poisoned detector may fail to detect a dangerous level of combustibles. Implementation of automated or frequent manual sensitivity check-up in a conventional detector is usually cost-prohibitive.
Conventional catalytic bead sensors can be made more or less selectively sensitive to some groups of gases, which is generally achieved by choosing a specific temperature setting of the catalytic bead. This temperature selection technique may not be effective at discriminating between different compositions with two or more combustible gases while using a single sensor and at one fixed temperature. In particular, catalytic bead sensors are practically unable to discriminate between carbon monoxide (CO) and hydrogen (H2) gases. Usually CO and H2 both start catalytically burning at nearly the same “low” catalyst temperature and are practically indistinguishable based on the temperature set-up of the bead(s). Accordingly, conventional catalytic detectors tend to be ineffective at: measuring low combustible gases concentrations, e.g., concentrations below 500 ppm (parts per million) over an extended time period, maintaining a stable “zero” without drifting over a period of years, and distinguishing between combustible gases, e.g. between CO and H2.